1. The Field of the Invention
The present invention relates generally to optical transmitters. More specifically, the present invention relates to active matching electro-optic transducer driver circuits that have high efficiency and are operable with low supply voltages.
2. Background and Relevant Art
Computing and networking technology have transformed our world. As the amount of information communicated over networks has increased, high speed transmission has become ever more critical. Many high speed data transmission networks rely on optical transceivers and similar devices for facilitating transmission and reception of digital data embodied in the form of optical signals over optical fibers. Optical networks are thus found in a wide variety of high speed applications ranging from as modest as a small Local Area Network (LAN) to as grandiose as the backbone of the Internet.
Typically, data transmission in such networks is implemented by way of an optical transmitter (also referred to as an electro-optic transducer), such as a laser or Light Emitting Diode (LED). The electro-optic transducer emits light when current is passed through it, the intensity of the emitted light being a function of the current magnitude. An electro-optic transducer driver generates the appropriate magnitude of current to pass through the electro-optic transducer to generate the appropriate amount of optical intensities corresponding to the data being transmitted.
In order to assert one binary value, a relatively low current (called herein “IBIAS”) is passed through the electro-optic transducer to thereby cause a relatively low optical power level to be transmitted onto the optical fiber. In order to assert the opposite binary value, a relatively high current is passed through the electro-optic transducer to thereby cause a relatively high optical power level (e.g., IBIAS plus a maximum modulation current called herein “IMOD”) to be transmitted onto the optical fiber. Accordingly, by superimposing a modulation current (that varies between zero and IMOD) upon the bias current, an appropriate sequence of bits may be transmitted.
FIG. 2 illustrates a driver-transducer circuit 200 that includes an electro-optic transducer 201 in the form of a specially manufactured diode. Methods for fabricating electro-optic transducer 201 in the form of a diode are well known in the art. The optical power transmitted by the electro-optic transducer 201 is approximately proportional to the amount of current passed through the electro-optic transducer 201 for the frequency range of interest.
The remaining circuitry 210 shown in FIG. 2 other than the electro-optic transducer 201 is an electro-optic transducer driver. The electro-optic transducer driver 210 applies the appropriate current through the electro-optic transducer 201 depending on the data. In the illustrated embodiment, the electro-optic transducer 210 is what is referred to as “DC-coupled” to the electro-optic transducer 201.
Specifically, a bias current source 211 draws a bias current IBIAS through the electro-optic transducer 201. In addition, a modulation current source 212 draws the maximum modulation current IMOD through either the bipolar transistor 221, or the bipolar transistor 222, or through both of the bipolar transistors 221 and 222 in a split manner. The amount of modulation current IMOD drawn through the electro-optical transducer 201 depends on the differential data signals DATA and DATA! applied at the base terminal of the corresponding bipolar transistors 221 and 222.
The DC-coupled circuitry 200 of FIG. 2 is advantageous in that the modulation current is drawn completely through the electro-optic transducer 201 through the bi-polar transistor 222. Accordingly, the circuit 200 is highly efficient. Furthermore, the DC-coupled circuit 200 may operate on a relatively low supply voltage of, for example, 3.3 volts. However, in high-frequency applications, the electro-optic transducer 201 is required to be physically close to the electro-optic transducer driver 210. Thus, heat generated by the electro-optic transducer driver 210 may be easily transferred to the electro-optic transducer 201. Some electro-optic transducers, such as lasers, are particularly sensitive to temperature in that the efficiency at which they convert electricity to light is highly temperature dependent. Furthermore, the speed of the laser is degraded as the temperature increases. Thus, either a lower level of performance must be expected of this configuration, or great expense is taken to provide an effective way of dissipating heat from the electro-optic transducer 201 despite the close proximity to the electro-optic transducer driver 210 that acts as a heat source.
FIG. 3 illustrates another conventional driver-transducer circuit 300. In this circuit, the modulation current of the electro-optic transducer driver 310 is “AC-coupled” to the electro-optic transducer 301. A bias current source 311 supplies the bias current IBIAS plus IMOD/2 through the electro-optic transducer 301. A modulation current source 312 causes modulation current to pass through the electro-optic transducer 301 through AC-coupling capacitors 325A and 325B.
Specifically, the modulation current source draws 1/n times (where “n” is the AC coupling coefficient) the maximal modulation current IMOD in a split manner through the bipolar transistors 321 and 322. The amount of current drawn through resistor 323 and through bipolar transistor 321 depends on data signal DATA and DATA!. Accordingly, the amount of current drawn through source resistor 324 (having resistance RS) and bipolar transistor 322 depends on data signal DATA and DATA! as well, since the sum of current drawn through both bipolar transistors 321 and 322 remains constant at IMOD/n.
The amount of current drawn through bipolar transistor 322 may thus vary from zero to IMOD/n, depending on the data signal DATA. Conversely, the current drawn through bipolar transistor 321 may vary from zero to IMOD/n as well, in a complementary manner to the current drawn through bipolar transistor 322. The resulting differential current is AC-coupled through capacitors 325A and 325B, through corresponding transmission lines 326A and 327B, and through corresponding load resistors 327A and 327B (each having resistance RL) so that only the fraction equal to the AC-coupling coefficient “n” of the differential current passing through bipolar transistors 322 and 321 is provided through the electro-optic transducer 301. Therefore, the modulation current provided through the electro-optic transducer 301 varies from zero to IMOD, depending on the data signal DATA.
The AC-coupled driver-transducer circuit 300 of FIG. 3 is advantageous in that the electro-optic transducer 301 may be located at some distance from the electro-optic transducer driver 310. Accordingly, heat generated by the electro-optic transducer driver 310 is not transferred as much to the electro-optic transducer 301. Furthermore, heat control units such as Thermo Electric Coolers (TECS) may more easily be affixed in effective proximity to the transducer without competing as much for space with the electro-optic transducer driver 310. Thus, the AC-coupled driver-transducer circuit 300 of FIG. 3 permits for more efficient and cost-effective control of temperature.
However, the AC-coupled driver-transducer circuit 300 does have a significant disadvantage. The driver circuit 310 in the AC-coupled driver-transducer circuit 300 must draw more modulation current than the driver circuit 210 in the DC-coupled driver-transducer circuit 200 of FIG. 2. For instance, the modulation current drawn by driver circuit 310 is IMOD/n, where “n” (the coupling coefficient) is less than one, and is ideally around 50% for optimal performance. Accordingly, the power dissipation of the driver circuit 310 in the AC-coupled configuration is higher than that in the DC-coupled configuration. This increased power usage increases the cost of operating the driver circuit, and also results in the driver circuit generating more heat. Heat may have an adverse impact on the performance of the driver circuit, and may result in reducing the operable lifetime of the driver circuit.
As an additional disadvantage, the presence of the source resistors 323 and 324 in the path between voltage sources means that the driver circuit 310 might operate using higher supply voltages. The driver circuit 210 of the DC-coupled configuration may operate at 3.3 volts, whereas the driver circuit 310 of the AC-coupled configuration may use supply voltages of 5 volts. The industry trend is to reduce the power dissipation either by reducing the required voltage supply and/or reducing current consumption in order to increase the density of optical transceivers that can be installed in a given space such as, for example, a switch box.
FIG. 4 illustrates a conventional active matching driver-transducer circuit 400. In this circuit 400, the driver 410 is differentially AC coupled to the transducer 401 through the capacitors 425A and 425B, the transmission lines 426A and 426B, and the load resistors 427A and 427B. Active matching makes it possible to deliver close to 100% of IMOD to the load RL and transducer 401, thereby improving the power efficiency of the circuit while maintaining good source-load matching and high-frequency performance. Unfortunately, in order to get a high bandwidth between the driver 410 and the transducer 401, the source and load resistors should have a sufficiently high resistance. Such resistances may typically be between twenty and fifty ohms. Accordingly, the way the transistors and resistors are stacked between the high and low voltage supplies, a relatively high voltage supply difference of 5 volts is typically used for this configuration.
Accordingly, what would be advantageous is an active matching driver-transducer circuit that has high AC coupling efficiency, and that may operate at a lower supply voltage.